Metallic nanoshells on porphyrin-stabilized emulsions

ABSTRACT

Metal nanostructures formed by photocatalytic interfacial synthesis using a porphyrin-stabilized emulsion template and the method for making the nanostructures. Catalyst-seeded emulsion droplets are employed as templates for hollow-nanoshell growth. The hollow metal nanospheres may be formed with or without inclusions of other materials.

This application is a divisional application of the prior-filed U.S.nonprovisional patent application Ser. No. 11/760,850, filed on Jun. 11,2007, updated as U.S. Pat. No. 8,075,664 B1, and claims priority benefittherefrom. This prior-filed application is hereby incorporated byreference.

The United States Government has rights in this invention pursuant toDepartment of Energy Contract No. DE-AC04-94AL85000 with SandiaCorporation.

BACKGROUND OF THE INVENTION

This invention relates to metallic hollow metallic nanostructures andmicrostructures. Microstructures and nanostructures having hollowinteriors, for example, nanoshells, have many potential applicationsbecause of their increased surface areas, low density, low materialcost, and sometimes special optical properties. Most previous nanoshelland microshell structures have been synthesized using multiple bottom-upprocesses by coating a hard core template followed by removing the coretemplate through etching. For example, Kim et al. have synthesizedpalladium hollow spheres using silica spheres as a template (S.-W. Kim,M. Kim, W. Y. Lee, and T. Hyeon, “Fabrication of Hollow Palladiumspheres and Their Successful Applications to the RecyclableHeterogeneous Catalyst for Suzuki Coupling Reactions,” J. Amer. Chem.Soc. Vol. 124 (2002) pp. 7642-7643). The surfaces of the silica sphereswere functionalized with mercaptopropylsilyl (MPS) groups. The palladiumprecursor, palladium acetylacetonate, was then adsorbed onto thesurfaces of the MPS-functionalized silica spheres. ThePd²⁺-adsorbed-MPS-functionalized silica spheres were heated at 250 C for3 hours to obtain Pd metal-coated spheres. The silica sphere was removedby HF etching.

Hollow nanoshells of gold have been prepared by leaching out silverchloride from AU_(shell)/(AgCl+horseradish peroxidase)_(core)nanoparticles with dilute ammonia solution (R. Kumar, A. N. Maitre, P.K. Patanjali, and P. Sharma, “Hollow gold nanoparticles encapsulatinghorseradish peroxidase,” Biomaterials, vol. 26 (2005), pp. 6743-6753).

Galvanic replacement processes where a sacrificial metal template isused have also been reported. Reaction of Pd (NO₂)₂ or Pt(CH₃COO)₂ withAg nanocrystal templates has produced Pd and Pt nanoshells (Y. Sun, B.T. Mayers, and Y. Xia, “Template-Engaged Replacement Reaction: AOne-Step Approach to the Large-Scale synthesis of Metal Nanostructureswith Hollow Interiors,” Nano Letters vol. 2 (002) pp. 481-485). Theshape of the nanocrystal template is reproduced in the nanoshellstructure. Liang et al. have synthesized Pt hollow nanospheres byexploiting the replacement reaction between Co nanoparticles and H₂PtCl₆(H.-P. Liang, H.-H. Zhang, J.-S. Hu, Y.-G. Guo, L.-J Wan, and C.-L. Bai,“Pt Hollow Nanospheres: Facile Synthesis and Enhanced Electrocatalysts,”Angew. Chem. Int. Ed. Vol. 43 (2004) pp. 1540-1543). Co nanoparticlesare oxidized to cobalt ions when the solution of Co nanoparticles isadded to a H₂PtI₆ solution. The reaction continues until the Co iscompletely consumed. The Pt shell is incomplete and porous.

Emulsion droplets have been used as coating templates for synthesis ofhollow spheres of oxides and semiconductors. These previousemulsion-based methods did not employ photocatalytic interfacialsynthesis of metallic nanoshells or microshells using aporphyrin-stabilized emulsion template.

Schacht et al. have combined long-range oil-in-water emulsion andoil-in-water interface physics with the shorter range cooperativeassembly of silica and surfactants at the oil-water interface to createordered composite mesostructured phases that are also macroscopicallystructured (S. Sachact, Q. Huo, I. G. Voign-Martin, G. D. Stucky, and F.Schuth, “Oil-Water Interface Templating of Mesoporous MacroscaleStructures,” Science, Vol. 272, (1996) pp. 768-771).

A related patent application that does not employ emulsions to formshells is John A. Shelnutt, Yujiang Song, Eulalia F. Pereira, and CraigJ. Medforth, “Dendritic Metal Nanostructures,” U.S. patent applicationSer. No. 10/887,535 filed Jul. 8, 2004. It is incorporated herein byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate some embodiments of the present inventionand, together with the description, serve to explain the principles ofthe invention.

FIG. 1 presents a schematic representation of one mechanism forformation of metal nanoshells at the surface of the porphyrin-stabilizeddispersed phase of an emulsion.

FIG. 2 illustrates one embodiment of a method for forming metalnanoshells using Pt as the seed metal and Pt or Pd as the shell metal.

FIG. 3 presents transmission electron micrographs (TEMs) of Ptnanoshells prepared using 0.125 vol. % benzene in water emulsionstabilized by SnP18. The concentrations of Pt(II) used for preparing thenanoshells are (a) 1 mM, (b) 0.5 mM, and (c) 0.25 mM.

FIG. 4 presents a TEM of the mixture of Pd nanoshells and Pd dendritesmade without Pt seeding using 0.5 vol. % chloroform in water emulsionsstabilized by SnP18.

DETAILED DESCRIPTION OF THE INVENTION

This invention comprises metal nanostructures formed by photocatalyticinterfacial synthesis using a porphyrin-stabilized emulsion template andthe method for making the nanostructures. Catalyst-seeded emulsiondroplets are employed as templates for hollow-nanoshell growth. Thehollow metal nanospheres may be formed with or without inclusions ofother materials.

Embodiments of this invention are based on a photocatalytic reduction ofmetal ions at the interface between the dispersed and continuous phasesof an emulsion. The metal reduction is photocatalyzed by ametallo-lipoporphyrin that resides at the water/organic liquid (oil)interface and can function as a surfactant that stabilizes the emulsionand as a photocatalyst. The photocatalytic reduction of metal ionsproduces a surface distribution of catalytic metal seed particles.Subsequent growth of a metallic sphere at the surface can beautocatalytically or photocatalytically promoted. FIG. 1 illustrates anembodiment of the invention. The size of the shell is controlled by thesize of the emulsion droplet and the wall thickness is controlled byboth the metal ion availability and the oil droplet concentrations. Aninclusion particle may be incorporated within the shell. A processflowchart is presented for one embodiment in FIG. 2.

The lipoporphyrin has both hydrophobic and hydrophilic characteristics.Consequently, the lipoporphyrin resides at the oil/water interface ofthe emulsion droplet. Metal nanoshells or microshells are formed at thisinterface by reactions that are photocatalyzed and/or autocatalyzedafter photocatalytic seed-metal production. For this invention, the termnanoshell is defined to include shell diameters ranging up toapproximately 10 micrometers. The size distribution of the shells isdetermined by the size distribution of the droplets of the emulsion. Asthe volume ratio of the organic-liquid component of the emulsion isreduced, the average size of the metal shells is decreased. For example,in embodiments involving benzene, the emulsion before irradiation isobserved to turn from yellow-milky to yellow-transparent when it isdiluted to 0.25 vol. % or lower. At volume ratios below 0.25%, most ofthe micron size droplets are gone and the emulsion becomes ananoemulsion, which appears transparent. The stabilizing effects of thelipoporphyrin at the oil-water interface allows the continuing presenceof the nanodroplets at volume ratios well below the normal solubilitylimit of benzene in water. The organic phase does not diffuse totallyinto the water phase, and the emulsion is sufficiently stable to survivethe metal-growing period. A variety of organic liquids may be used asthe dispersed phase. The oils phase should be able to solubilize thehydrophobic substituents of the photocatalytic porphyrin. Examples ofsuitable organic liquids include but are not restricted to benzene,chloroform, and dichloromethane, and other organic solvents withappropriate solubility in water to allow stable nanoemulsion formation.Embodiments have been demonstrated using benzene, chloroform, anddichloromethane. Platinum nanoshells have been prepared using a 2 vol. %dichloromethane (containing 1 mM SnP18)-in-water emulsion and using a0.5 vol. % chloroform (containing 1 mM SnP18)-in-water emulsion. Otherpolar solvents which can form the emulsion and dissolve a sufficientamount of the lipoporphyrin to provide a sufficient concentration in theinterface to catalyze the reduction may be used. The photocatalyticporphyrin comprises a hydrophilic head and a hydrophobic tail, asdiscussed below.

The sizes of droplets in the emulsion will determine the sizes of thenanoshells. Increasing the volume fraction of the organic liquid willincrease the size of the metal nanoshells. For example, in an embodimentemploying 0.5 vol. % benzene in water, the product consisted of hollowplatinum structures with a range of sizes and morphologies and with asignificant quantity of micron-sized structures present. At 0.25 vol. %benzene, relatively few micron-sized shells are observed in scanningelectron micrographs (SEMs) and most of the product consisted ofspherical Pt shells between 100 and 400 nm in diameter. With 0.125 vol.% benzene, the size of the nanoshells was further reduced and most ofthe product consisted of nanoshells between 50 and 150 nm in diameter.Good reproducibility was obtained for the emulsions prepared withbenzene concentrations below approximately 0.22 vol. %.

In some embodiments, Sn(IV) meso-tetra(N-octadecyl-4-pyridyl)porphyrin(SnP18) was dissolved in dichloromethane or chloroform or suspended inbenzene using mild sonication. The concentration of SnP18 in the organicliquid as described herein is calculated based on the SnP18 amount inthe organic liquid regardless of whether the SnP18 is dissolved orsuspended. An oil-in-water emulsion was prepared by injecting 0.02 mL ofSnP18-containing organic liquid into 2 mL of water followed by mildsonication for approximately 2 minutes to produce a 1 vol. % emulsion. Atransparent yellow solution was produced when dichloromethane was usedas the organic (oil) phase because the oil-phase volume ratio was veryclose to its solubility in water (2 vol. %). For chloroform and benzene,a milky yellow emulsion was produced (solubility in water is 0.5 vol. %for chloroform and 0.22 vol. % for benzene). The freshly preparedemulsion was further diluted using distilled water to a desired volumeratio. When the emulsion was diluted to a volume ratio near thesolubility of the organic phase, the emulsion became transparent.

In forming the emulsion, it is desirable to achieve a thermodynamicallystable or metastable state. This is facilitated by the organic liquidbeing slightly soluble water. To make microemulsions or nanoemulsionsfrom thermodynamically unstable macroemulsions, a dilution technique isuseful in some embodiments where the initial organic concentration doesnot produce the stable or metastable transparent microemulsion ornanoemulsion. Following formation of the macroemulsion using standardtechniques, such as sonication, forcing the oil/water mixture through anultrafine mesh at high pressure, and other methods of mixing, dilutionby addition of water until the organic liquid is near its solubilitylimit causes shrinkage of the droplets until the thermodynamicallystable curvature is achieved. The emulsion turns from milky totransparent near this transition. Depending on the organic liquid andthe lipoporphyrin, this should occur at a concentration between 10 timesand 1/10 times the solubility limit of the organic liquid in water.

The selection of a suitable lipoporphyrin is guided by the need to forit to be both sufficiently hydrophilic and hydrophobic to reside ineffective quantities at the oil/water interface. An effective quantityboth stabilizes or assists in stabilizing the emulsion and serves as aphotocatalyst for the formation of the Pt seeds in the region of theoil-water interface. A convenient method for achieving this is toincorporate substituents that have both a hydrophilic segment and ahydrophobic segment. Examples of suitable hydrophilic segments includebut are not restricted to hydroxyl, amide, carboxyl, pyridyl, sulfonate,phosphate, amine, and other hydrophilic groups. Examples of suitablehydrophobic segments include but are not restricted to hydrocarbonscontaining 5 to 20 carbon atoms. For example, with SnP18, thesubstituent group is the N-octadecyl-pyridinyl group, where theoctadecyl group acts as the hydrophobic tail while the pyridyl group hasa positive charge and, along with the charged porphyrin ring system, ispart of the hydrophilic head group.

Shell thickness can be controlled by varying the Pt salt concentrations.In some embodiments, a 0.125 vol. % benzene/water emulsion was used withPt(II) concentrations of 1.0 mM, 0.5 mM, and 0.25 mM Pt(II) ionconcentrations. Nanoshells from these reactions are shown in FIGS. 3 a,3 b, and 3 c, respectively. The average wall thickness decreased form 40nm to 20 nm when the initial Pt(II) concentration was decreased from 1mM to 0.5 mM. The very thin Pt shell is visible in FIG. 3 c forstructures made from the low initial Pt(II) concentration of 0.25 mM.

The photocatalytic reduction seeding of platinum salts by themetallolipoporphyrin, as represented by the example SnP, is accomplishedin the presence of visible light and an electron donor (ED) species.Suitable electron donor species include ascorbic acid, triethanolamine,ethylenediamine tetraacetic acid and the salts thereof, ethanol, andmethanol. The SnP photoreaction is a reductive photocatalytic cycle,described by the following simplified equations:SnP+hμ→SnP*SnP*+ED→SnP⁻.+ED_(ox)2SnP⁻.+Pt²⁺→2SnP.+Pt⁰

After it is produced, the Pt seed can serve as a catalyst and growautocatalytically into the mature nanostructure. In some embodiments ofthis invention, the photocatalytic SnP molecules align on the emulsiondroplet surface. The platinum seeds are produced at the oil-waterinterface through a photocatalytic process by exposing the droplets tolight. The seeds can act as catalytic centers and a closed shell can beformed through an autocatalytic reaction mechanism. Continuing growthcan increase the shell wall thickness, depending on the Pt(II)concentration for a particular reaction.

A very thin Pt shell, possibly formed in the photocatalytic stagefollowed by limited autocatalytic growth of Pt islands, is observed atlow initial Pt(II) concentration (0.25 mM, FIG. 3 c). Without the lightexposure, the Pt(II) can be also reduced by an electron donor, such as,for example, ascorbic acid, but the reaction time is much longer and theproducts are much less uniform. The reaction without the photocatalyticporphyrin usually takes hours to produce visually detectable product,while only minutes are require using the photocatalyst. In addition, theplatinum formed without the photocatalyst occurs as irregular aggregatesinstead of shells. Thus, the initial photocatalytic reduction at theoil-water interface of the droplet plays an important role in formingthe shell structure.

Incorporation of an inclusion particle of a material that is solubilizedin the organic liquid of the dispersed phase is part of someembodiments. One such embodiment is demonstrated in the formation of amagnetic nanoshell product using a porphyrin-stabilized emulsion.Particles of Fe₃O₄ were recovered from Ferrofluid to providenanoparticles with surfaces modified to allow ready solubility in theorganic phase. Other embodiments based on other particles (with orwithout bound solubilizer molecules) that are soluble in the dispersedorganic phase can similarly be incorporated within the metal nanoshellsor microshells formed by execution of this method. The magnetitenanoparticles contained in Ferrofluid were recovered by mixing 1 mL ofFerrofluid and 2 ml of benzene and adding 3 mL of ethanol to precipitatethe magnetic particles. The mixture was centrifuged at 4000 rpm for 10minutes, the supernatant was removed, and the precipitate was air-driedfor approximately 20 minutes. The precipitate was re-suspended in 2 mLof benzene by mild sonication and 0.2 mL of the suspension was mixedwith 0.2 mL of a suspension of SnP18 in benzene (3 mM SnP18). A 0.2 mLaliquot of this suspension (1.5 mM SnP18) was diluted with distilledwater (20 mL) and sonicated to produce a 1 vol. % emulsion of benzene inwater that was 15 μM SnP18. The magnetite-containing emulsion dropletswere then used as nanoshell templates. Such core-shell nanostructuresthat contain a magnetic inclusion can be magnetically manipulated. Thisfeature can be very useful for easy recovery for reuse of nanoshellsthat have been used as catalysts or for other applications.

A variety of inclusions may be employed in various embodiments of thisinvention. Many types of particles that are water insoluble but solublein the organic liquid may be used. The size of the particle that may beincluded is determined in part by the emulsion droplet size.Nanoparticles size is largely determined by the inner volume of thedroplet. Examples of suitable particles include but are not restrictedto Fe₃O₄, Fe₂O₃, TiO₂, semiconductor quantum dots, Au particles, Agparticles, FePt particles, Ni particles, Co particles, and many alloyedparticles where the particle is either inherently soluble in the organicliquid or is made soluble by surface modification with a material thatfacilitates solubilization. Organic dyes, organic fluorescent molecules,and other organic materials that are soluble in the organic liquid butminimally soluble in water may also be used as inclusions.

In some embodiments that are typical of photocatalytic platinumnanoshell synthesis, 2 mL of diluted emulsion solution was transferredto a reaction vial. A Pt (II) solution was prepared at a concentrationof 20 mM by dissolving K₂PtCl₄ in water at room temperature and agingthe mixture. In some embodiments, the mixture was aged for approximatelyone day before use. A volume of 0.15 M ascorbic acid solution was addedto the emulsion solution. The pH of the reaction solution wasapproximately 3 due to the ascorbic acid. The concentration of Pt(II) invarious embodiments was between 1 mM and 0.025 mM and the concentrationof ascorbic acid was approximately 7.5 times that of Pt(II). The vialwas irradiated with incandescent light (800 nmol/cm²-sec) forapproximately 30 min. The reaction solution turned gradually to a blacksuspension, which is evidence of the conversion of the Pt(II) in thesolution to Pt atoms.

For the synthesis of magnetic platinum nanoshells in one embodiment, 20mL of 1 vol. % magnetic benzene nanodroplets produced as described abovewas diluted with water (60 mL) to produce a template mixture containingapproximately 0.25% benzene (3.7 μM SnP18). To this solution was added 3mL of aged platinum complex (20 mM) and 3 mL of ascorbic acid (150 mM).The mixture was irradiated with incandescent light for approximately 40minutes. The solution was centrifuged at 4000 rpm for approximately 10minutes, and the supernatant liquid was removed. Distilled water (30 mL)was added to the centrifuge tube, and the precipitate was re-suspended.This purification procedure may be repeated.

For the synthesis of palladium nanoshells in one embodiment, an aqueoussolution of 10 mM K₂PdCl₄ and 75 mM triethanolamine (TEA) was combinedwith a Pt-seeded emulsion. In one embodiment, 0.01 mL of 20 mM Pt(II)complex solution and 0.1 mL of 0.15 M ascorbic acid was combined with2.0 mL of 0.5 vol. % chloroform (containing 1 mM SnP18) in water. Themixture was irradiated with light (800 nmol/cm²-sec) for approximately10 min to produce the Pt-seeded emulsion. Subsequently, 0.2 mL of thePd(II)/TEA solution was added to the Pt-seeded emulsion. The yellowemulsion gradually turned black over the course of several minutes.

For formation of Pd-coated nanoshells as the predominant product, thepresence of a Pt seed layer at the oil/water interface is desirable. Theeffect of preseeding with Pt and the effect of TEA in the formation ofPd nanoshells has been studied. With 2 mL of an emulsion of 0.5 vol %chloroform containing 1 mM SnP18, 0.01 mL of 20 mM Pt(II) and 0.1 mL of0.15 M ascorbic acid were added and the solution was illuminated for 10min. Addition of 0.2 mL of (0.75 mM TEA+10 mM Pd(II)) produced Pdnanoshells. If 0.1 mL of 0.2 M ascorbic acid and 0.2 mL of (0.75 mMTEA+10 mM Pd(II)) were added to the emulsion without Pt seeding, amixture of Pd nanoshells and Pd dendrites formed. If 0.1 mL of 0.15 Mascorbic acid and 0.1 mL of 20 mM Pd(II) were added (no TEAstabilization), Pd dendrites instead of Pd shells were produced. FIG. 4presents a transmission electron micrograph (TEM) of the unseededTEA-stabilized case where both Pd nanoshells and Pd dendrites areproduced.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

What is claimed is:
 1. A nanoparticle comprising: an emulsion droplet; aplurality of lipoporphyrin molecules, wherein a hydrophobic substituentof the lipoporphyrin molecule is contained within a surface region ofthe emulsion droplet; a metallic seed shell of diameter approximatelyequal to or greater than a diameter of the emulsion droplet andsubstantially surrounding the emulsion droplet; a metallic outer shellatop the metallic seed shell, wherein the metallic seed shell and themetallic outer shell form a hollow nanoshell surrounding the emulsiondroplet; and a second metallic outer shell atop the metallic outershell, wherein the metallic outer shell has a first metallic compositionand the second metallic outer shell has a second metallic compositionthat is not identical to the first metallic composition.
 2. Thenanoparticle of claim 1, wherein the emulsion droplet is substantiallythermodynamically stable.
 3. The nanoparticle of claim 1, wherein theemulsion droplet comprises an organic liquid possessing a solubility inwater suitable for forming an emulsion that is stable in water for atime required to form the metallic seed shell.
 4. The nanoparticle ofclaim 1, further comprising an inclusion particle that is solubilized inthe emulsion droplet.
 5. The nanoparticle of claim 4, wherein theinclusion particle is selected from the group consisting essentially ofFe₃O₄, Fe₂O₃, TiO₂, semiconductor quantum dots, metal particles, metalalloy particles, and solubilized versions thereof made soluble bysurface modification with a material that facilitates solubilization. 6.The nanoparticle of claim 1, wherein the metallic seed shell is Pt. 7.The nanoparticle of claim 1, wherein the metallic outer shell isselected from the group consisting of Pt, Pd, Au, and Ag.
 8. Thenanoparticle of claim 1, wherein the emulsion droplet comprises amaterial selected from the group consisting of benzene, chloroform,dichloromethane, and an organic oil capable of solubilizing thehydrophobic substituent of the lipoporphyrin molecule.
 9. Thenanoparticle of claim 1, wherein the lipoporphyrin molecule comprises ahydrophilic head and a hydrophobic tail.
 10. The nanoparticle of claim9, wherein the hydrophilic head comprises a hydrophilic segment selectedfrom the group consisting of a hydroxyl group, an amide group, acarboxyl group, a pyridyl group, a sulfonate group, a phosphate group,and an amine group.
 11. The nanoparticle of claim 9, wherein thehydrophobic tail comprises a hydrophobic moiety selected from the groupconsisting of hydrocarbon units containing between 5 and 20 carbonatoms.
 12. The nanoparticle of claim 1, wherein the lipoporphyrinmolecule is a metallo-lipoporphyrin.
 13. The nanoparticle of claim 12,wherein the metallo-lipoporphyrin comprises a photocatalyticmetalloporphyrin unit.
 14. The nanoparticle of claim 13, wherein thephotocatalytic metalloporphyrin unit is selected from the groupconsisting of a tin(IV) lipoporphyrin and tin(IV)meso-tetra(N-octadecyl-4-pyridyl)porphyrin chloride.
 15. Thenanoparticle of claim 1, where the lipoporphyrin molecule is selectedfrom the group consisting of Sn(IV)meso-tetra(N-octadecyl-4-pyridyl)porphyrin and salts thereof.
 16. Thenanoparticle of claim 5, wherein a metal of the metal particles isselected from the group consisting of Au, Ag, Ni, and Co and a metal ofthe metal alloy particles is FePt.